The present invention relates generally to the field of microwave circuits, and more particularly to integrated thick film RF and microwave microcircuit modules, and even more particularly to the dissipation of heat generated by such modules.
Microwaves are electromagnetic energy waves with very short wavelengths, typically ranging from a millimeter to 30 centimeters peak to peak. In high-speed communications systems, microwaves are used as carrier signals for sending information from point A to point B. Information carried by microwaves is transmitted, received, and processed by microwave circuits.
Packaging of radio frequency (RF) and microwave microcircuits has traditionally been very expensive and has required very high electrical isolation and excellent signal integrity through gigahertz frequencies. Additionally, integrated circuit (IC) power densities can be very high. Microwave circuits require high frequency electrical isolation between circuit components and between the circuit itself and other electronic circuits. Traditionally, this need for isolation was resulted in building the circuit on a substrate, placing the circuit inside a metal cavity, and then covering the metal cavity with a metal plate. The metal cavity itself is typically formed by machining metal plates and then connecting multiple plates together with solder or an epoxy. The plates can also be cast, which is a cheaper alternative to machined plates. However, accuracy is sacrificed with casting.
One problem attendant with the more traditional method of constructing microwave circuits is that the method of sealing the metal cover to the cavity uses conductive epoxy. While the epoxy provides a good seal, it comes with the cost of a greater electrical resistance, which increases the loss in resonant cavities and increases leakage in shielded cavities. Another problem with the traditional method is the fact that significant assembly time is required, thereby increasing manufacturing costs.
Another traditional approach to packaging RF/microwave microcircuits has been to attach gallium arsenide (GaAs) or bipolar integrated circuits and passive components to thin film circuits. These circuits are then packaged in the metal cavities discussed above. Direct current feed-through connectors and RF connectors are then used to connect the module to the outside world.
Still another method for fabricating an improved RF microwave circuit is to employ a single-layer thick film technology in place of the thin film circuits. While some costs are slightly reduced, the overall costs remain high due to the metallic enclosure and its connectors, and the dielectric materials typically employed (e.g., pastes or tapes) in this type of configuration are electrically lossy, especially at gigahertz frequencies. The dielectric constant is poorly controlled at both any specific frequency and as a function of frequency. In addition, controlling the thickness of the dielectric material often proves difficult.
A more recent method for constructing completely shielded microwave modules using only thick film processes without metal enclosures is disclosed by Lewis R. Dove, et al. in U.S. Pat. No. 6,255,730 entitled xe2x80x9cIntegrated Low Cost Thick Film RF Modulexe2x80x9d.
Heat dissipation from integrated circuits and other devices in high frequency microcircuits is an especially difficult problem. In order to increase heat transfer from those microcircuit devices having high heat dissipation, the devices are often attached directly to heat sinks, also referred to as heat spreaders or heat pedestals. However, lower thermal conductivity often precludes attachment to the organic or ceramic substrate typically used in such circuits.
Thus, when high power integrated circuits or other high power devices are used in an integrated Thick Film Microwave Module, a hole is usually cut in the ceramic substrate to accommodate a metallic heat sink. This cut breaks the electrical isolation provided by a ground plane typically located on top of the substrate. This break in electrical isolation is undesirable for microwave applications as they typically require very high electrical isolation. Breaks in the ground plane result in the radiation of electromagnetic energy.
Thus, there is a need for a means of attaching heat sinks to devices in high frequency microcircuits without compromising the electrical isolation of the module.
In one embodiment, a heat sink apparatus that provides electrical isolation for an integrally shielded, electronic circuit comprises a substrate having a first hole extending between a first and second sides of the substrate, a conductive layer attached to the second side, an electrically and thermally conductive heat sink having a protrusion, wherein the heat sink is attached to the first side of the substrate, and an electrically conductive plate having a second hole extending through the plate. The protrusion extends through the first hole and has a surface located at substantially the same level as that of the conductive layer. An electronic component is attachable to the protrusion surface. The plate is electrically coupled to the conductive layer and to the protrusion surface such that open space between the protrusion and the conductive layer is covered by electrically conducting area of the plate.
In another embodiment, a heat sink apparatus that provides electrical isolation for an integrally shielded, electronic circuit comprises a substrate having a first hole extending between a first and second sides of the substrate, a conductive layer attached to the second side, and an electrically and thermally conductive heat sink having a protrusion. The heat sink is attached to the first side of the substrate. The protrusion extends through the first hole and has a surface located at substantially the same level as that of the conductive layer. An electronic component larger than the protrusion surface is electrically connectable to the conductive layer and is electrically and thermally connectable to the protrusion surface such that open space between the protrusion and the conductive layer is covered by the electronic component.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.